Field of the Invention
This invention relates to a radiation image recording and read-out apparatus for exposing a stimulable phosphor to radiation in order to store a radiation image therein, scanning the stimulable phosphor with a stimulating ray to cause the stimulable phosphor to emit light in a pattern corresponding to the radiation image stored therein, and reading out the emitted light to obtain an electric image signal, and more particularly to such an apparatus in which a radiation image is stored in a stimulable phosphor layer formed on an endless belt.
Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultra-violet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to a stimulating ray such as visible light, light is emitted from the phosphor in proportion to the amount of radiation energy which was stored by the phosphor. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed, for instance, in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, there has been proposed a radiation image recording and reproducing system in which a radiation image of an object such as the human body is recorded and reproduced by the use of such a stimulable phosphor. Specifically, in this system a recording medium bearing thereon a stimulable phosphor layer is first exposed to radiation which has passed through an object in order to store a radiation image of the object in the stimulable phosphor layer, and then the stimulable phosphor layer is two-dimensionally scanned with a stimulating ray which causes it to emit light in a pattern corresponding to the stored radiation image. The light emitted from the stimulable phosphor layer upon stimulation thereof is detected photoelectrically and converted into an electric image signal, which is processed to reproduce the radiation image as a visible image on a recording medium such as a photosensitive material, a display system such as a CRT, or the like.
This system is advantageous over conventional radiography using a silver halide photosensitive recording material in that a radiation image can be recorded over a much wider radiation energy intensity exposure range. That is, it has been found that the intensity of light emitted from the stimulable phosphor upon stimulation thereof after it is exposed to radiation remains proportional to the energy of the radiation to which it was exposed for a very wide radiation energy intensity range. Accordingly, even if the energy intensity range of the radiation to which the stimulable phosphor is exposed varies substantially due to changes in the recording conditions, a visible radiation image independent of variations in the radiation energy intensity range can be obtained by choosing an appropriate gain when converting the light emitted from the phosphor upon stimulation into an electric signal.
In the radiation image recording and read-out apparatus, the recording medium bearing thereon the stimulable phosphor layer (herebelow referred to as "a stimulable phosphor recording medium") is used for temporarily storing the radiation image before forming a visible image on a final recording medium. Accordingly, the stimulable phosphor recording medium can be used repeatedly, and it is desired for economical reasons that the stimulable phosphor recording medium be used repeatedly.
Thus, we have proposed a radiation image recording and read-out apparatus in which the stimulable phosphor recording medium can be circulated and reused with a high efficiency. (See U.S. Pat. No. 4,543,479, for instance.)
The apparatus comprises a recording medium support, a stimulable phosphor recording medium which has a stimulable phosphor layer and is affixed to the recording medium support, an image recording section for exposing the recording medium to radiation which has passed through an object in order to store a radiation transmission image of the object therein, an image read-out section having a photoelectric read-out means which scans the recording medium storing therein the radiation image with a stimulating ray to cause it to emit light and reads out the light emitted from the recording medium to obtain an electric image signal, means for circulating the recording medium with respect to the image read-out section, and an erasing section which eliminates residual radiation energy on the recording medium after read out of the radiation image in the image read-out section and before another radiation image is recorded on the recording medium, the sections and means being incorporated into a unit. As the recording medium, a type comprising an endless belt-like base material with a plurality of stimulable phosphor layers formed thereon are very convenient. That is, the recording medium is passed around rollers and the like and conveyed in order to move the stimulable phosphor layers through the image recording section, the image read-out section and the erasing section in sequence.
We have further proposed a radiation image recording and read-out apparatus in which a recording belt comprising a stimulable phosphor layer formed on an endless belt-like base material is used as the stimulable phosphor recording medium, and which can be small in size and can effect highly accurate scanning of the recording belt with a stimulating ray in the image read-out section. (See U.S. Patent Application Ser. No. 143,919, now U.S. Pat. No. 4,847,499.) In this apparatus, the endless recording belt is passed through a pair of roller sections which are spaced from each other by a distance substantially equal to the length of one radiation image frame, and the image recording section is disposed so as to act on the recording belt between the roller sections. This arrangement permits one dimension of the apparatus to be reduced to substantially the length of one radiation image frame. Further, in the apparatus described above, a stimulating ray is caused to scan the recording belt on the peripheral surface of a roller. Since the recording belt runs most stably on the peripheral surface of the roller, the arrangement prevents wobbling of the recording belt and permits the stimulating ray to scan the recording belt accurately.
In the apparatus where an endless recording belt is used as the recording medium, the recording belt has to be conveyed during circulation in a stable way so that it does not become slack, or the like. For this purpose, it is preferred that a tension roller urged so as to impart tension to the recording belt be provided. However, though the recording belt can be conveyed accurately in a stable way during operation of the apparatus by virtue of the tension roller, the recording belt can deteriorate in a short time due to the tension constantly imparted thereto by the tension roller.